I think the best thing that a teacher can do, especially for children, is to give them two things:

First, the motivation and interest to learn. Make it fascinating! Children are by nature, curious, and want to learn more about the world. Be sure you are nurturing that curiosity, rather than extinguishing it as many school programs tend to do.

Second, help them study on their own! Give/show them any tools, games, or whatever interesting, fun things with which they can use or hear the language. Does he like movies? Why not show him some classic movies, such as Miyazaki's films, with subtitles? (<- that should also help his English reading comprehension!).

It's kind of hard to give advice, because you have to gauge his abilities, attention span, and motivation. I've known some young kids (and indeed, I was one of them), who, at that age, were actively reading and learning things on their own. So, be sure to give him credit for his abilities! I think a lot of time, adults try a bit too hard to "dumb things down" for kids. When teaching a group of kids, that is kind of required, but if you are teaching him one-on-one, be sure to assess his abilities and personality well! If he has a long or short attention span, if he reads and writes well, if he shows an interest in pursuing the language (or other interests) outside of class - all of these sorts of things will affect how best he will learn, and what methods you should use.

And, lastly, one piece of advice! With young kids, I find that some people focus far too much on memorization, or trying to cram specific knowledge in their heads. Unfortunately, that method is both ineffective, and damaging, I believe. Kids, and even adults, learn through experience. An immersive environment, and a curious mind is what helps them to do this. Rote memorization disconnects that which is being learned from any meaningful context, and is just plain boring. So, focus on what things you can do to get him interested in the language! Focus on fun ways to use it! And focus on ways that he can be exposed to the language. You would be surprised how much people, and especially kids, can pick things up on their own.

Of course, please take all of this advice with a grain -- nay, a salt shaker full of salt! I am only speaking from very limited experience of teaching a few abnormally bright kids informally, and outside of a classroom setting, and from what I remember about myself at that age. I think the key, at any rate, is to look at it from his perspective. Don't focus on how you should "teach," but how he "learns."

Anyway, those are just my thoughts!

By the way, why does he not speak the language? Do his parents? Is he living in the U.S.?If his parents speak it, and he has a fairly immersive environment at home, that will do wonders.